1. Field of the Invention
Example embodiments generally relate to a fixing device, and more particularly, a fixing device for fixing an image on a recording medium by applying heat and pressure.
2. Description of the Related Art
A related-art image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction printer having at least one of copying, printing, scanning, and facsimile functions, typically forms a toner image on a recording medium (e.g., a recording sheet) by electrophotography. Thus, for example, a charging roller uniformly charges a surface of a photoconductor. An optical writer emits a light beam onto the charged surface of the photoconductor according to image data to form an electrostatic latent image on the photoconductor. A development device develops the electrostatic latent image with toner to form a toner image. The toner image is then transferred from the photoconductor onto a recording sheet. Finally, a fixing device applies heat and pressure to the recording sheet bearing the toner image to fix the toner image on the recording sheet, thus forming an image on the recording sheet.
FIGS. 1A, 1B, 1C, and 1D illustrate a fixing device 1R as one example of such fixing device. The fixing device 1R includes a belt 101R, a fixing roller 100R, and a pressing roller 102R. The belt 101R is looped over the fixing roller 100R. The pressing roller 102R presses against the fixing roller 100R through the belt 101R to form a nip between the pressing roller 102R and the fixing roller 100R. When a recording sheet bearing a toner image passes through the nip, the fixing roller 100R and the pressing roller 102R apply heat and pressure to the recording sheet at the nip to fix the toner image on the recording sheet.
Over time, a length of the nip formed between the pressing roller 102R and the fixing roller 100R may vary due to change in diameter and hardness of the pressing roller 102R and the fixing roller 100R over time, resulting in an unstable quality of a fixed toner image.
FIG. 1A illustrates the fixing device 1R in which the fixing roller 100R has a default, normal diameter, and thereby the nip has a default, normal length. The diameter of the fixing roller 100R may change gradually over time or may change due to change in temperature of the fixing roller 100R as an amount of heat applied to the fixing roller 100R changes. For example, the temperature of the fixing roller 100R immediately after the fixing device 1R is warmed up is different from that of the fixing roller 100R after a number of recording sheets has continuously passed through the nip. When the diameter of the fixing roller 100R is changed, the fixing roller 100R has a shorter diameter and thereby the nip has a shorter length as illustrated in FIG. 1B.
FIG. 1C illustrates the fixing device 1R in which the fixing roller 100R has a default, normal hardness, and thereby the nip has a default, normal length. Yet the hardness of the fixing roller 100R may change over time. When the hardness of the fixing roller 100R is decreased, the fixing roller 100R is deformed, thus shortening the nip length as illustrated in FIG. 1D because, for example, when the hardness of an elastic member forming a surface layer of the fixing roller 100R is decreased, a tension applied by the belt 101R may deform portions of the elastic member on both sides near an entrance and an exit of the nip, causing the deformed portions of the elastic member to separate from the pressing roller 102R and thus shortening the nip length.
The change in the length of the nip may in turn cause the amount of heat and pressure applied to a recording sheet passing through the nip to fluctuate, resulting in an unstable quality of a fixed toner image.
Obviously, such unstable quality of a fixed toner image is undesirable, and accordingly, there is a need for a technology to provide a stable quality of a fixed toner image.